the two rates are the vert and horz (30 and 7865) these are for setting up the scope sweep speed.

I am looking at a factory manual, the scope readings do not match up, with the sams so can't really help. So you are back to checking for a pulse (should be two a vert rate pulse and a horz rate pulse both are for blanking.

you can trace the pulse from the flyback (should be pretty big at the horz rate, like 200v pp) thru a cap (.0047) thru the resistor (33k) thru the diode all the way to the transistor, and then see it at the emitter.

The best way to know for sure is to have a working set to compare to, but I realize that is not something that you have.

I would check the diode (easy since its already been clipped), and the transistor (again easy since it's socketed). if they check use the scope and see if you get both horz and vert blanking.

Okay, here's a pretty simple question about testing transistors. If I have an NPN and clip the positive lead to the base, should I set my meter to diode or 20M? When set to diode and test a transistor I have just sitting around, I get about the same numbers (580 - 620) on the base and collector. If I set it for 20M, I get 2.45 and 10.38.

diode check base to collector check it like a diode
diode check base to emitter check it like a diode
Ohm check emitter to collector should read open both ways (switch leads).

if if doubt just do a you tube search on checking transistors with DVM there are plenty of demos.

I use a analog meter that has a built in reversing switch in the resistance mode.

Got it. So now I must wonder why the transistor I replaced on the NOS horizontal board doesn't work in the board, but checks out good when tested. I've gone back and forth with it a few times in the board and it sure doesn't work. The transistor from the old horizontal board works perfectly.

I know I'll get it. I've made quite a bit of progress and gained quite a bit of knowledge in the past month.

Thanks

IF a transistor works in one board and not in the other in the same location, I would say the transistor is ok, I am a little confused on this, make sure you are testing correctly reverse the E to C leads, should read open both ways

BC open one way, low resistance when leads reversed.
BE open one way, low resistance when leads reversed.

you need to make sure you are doing this right. But I dont really understand why you are futzing around with the horz board. Check the diode in the blanking circuit.

The horizontal board gives me a known bad circuit that I can comfortable work with for practice.

The transistor from the NOS board is the exact same NPN as on the original board. I put the meter in diode mode and connected the positive to the base (the E, B and C are clearly marked on the metal tab). I connected the negative lead to the collector and got a reading. I then connected the negative to the Emitter and got a reading. I then connected the positive to the emitter and the negative to the collector and got nothing. I reversed the leads and got nothing.

I put that transistor back on the board, install the board and the TV won't fire up other than sound. If I take the transistor from the old board and put it in the new board, it works like a charm.

I guess I'll just let this one go. Since I seem to be doing it correctly, maybe there's something else wrong Maybe the sockets don't like the legs of this one (they are gold plated unlike the older, working one). Maybe it has a thing about flashy transistors.

Anyway, I'll just wait for the scope and my brother.

in the diode mode did you reverse the polarity after the Base to Emitter same for reverse the Base to collector? diode checks must be made both ways same on transistor, you must check base to emitter and base to collector BOTH ways.

Yes, I did the following:

Positive on base and negative on emitter and collector (readings on both)
Negative on base and positive on emitter and collector (no readings)
Negative on collector and positive on emitter (no readings)
Positive on collector and negative on emitter (no readings)

Those are all the possible combinations. Like I said, I have no idea why this one won't work, but I am confident I've tested it properly. Oh well, like I said. Maybe it's something about the socket.

those should be about .6v for a silicon transistor (when you got the reading).

DaveWM has you on the right track. Only thing I
will add is the 2 100K resistors are labeled 5% ( R201 & 206 )
and would be highly suspect. Also since you will
have a scope be sure there is NO crap on the 23.5v
( source #2 ). Should be pure DC.

73 Zeno:smoke:

What would be nice is if they would label the posts for connecting the scope like they do the ones for calibration. All the points necessary for APC, Brightness Limiting and such are marked. Scope points would be nice as well. I'll be going under the chassis to note all the points, note their positions on the strips and then noting their positions on the top side.

This should make life easier and I don't have to keep the set on its side while I test.

did you ever get anywhere on this problem? the main things to look at and check if you did not scope would be that diode and those two voltage divider resistors )100k each, think they were banded 5%

Well getting my brother over with his scope is proving to be a major task. I don't know when he'll have the time or desire.

I did manage to find NOS OEM transistors and the blanking diode. I also got my hands on the 1K, 33K and some 100K resistors, but I have a question about the 100Ks. The original 33K is a carbon composite (I believe from its shape and brown color case). The 100Ks look like standard film. I got my hands on the 33K replacement in the composite form. My question is about the 100Ks. I was given a few that are also composite, not film. Is it okay to use the composites as replacements? The wattage is the same. In the attached pics you can see the diode, 33K and two 100Ks in one and the 1K connected to the transistor in the other pic). Those are the ones I'm looking at changing.

So with the new transistor, diode and resistors I might just go ahead and replace them all. Of course this is after I replace the blue convergence coil that seized up. I managed to find an NOS replacement and it should be here Monday at the latest.

I don't see any problem with using metal films in that location. Be aware that even new carbon comps can be way off spec, always best to test. Tube sets are a bit more tolerance specs. if its marked 5% you should check before it goes in.

Just a warning, changing those parts may have NO effect on the problem, but its the only thing that comes to mind when you talk about vert bars commonly described as jail bars. The scope would let you see it before and after (the blanking) and may be enough to tell if its the problem at all, but even that is not assured since the waveforms may not be very reliable.

If I were in your shoes I would just change it and see. keep the old parts if by some chance the replacements make things worse (I don't see it happening but never hurts).

good luck with it.

Okay, the diode arrived today as well as some 33Ks. The 100Ks should be here on Monday.

Since I was ordering NOS parts, I went down the list of transistors and ordered what they had so I could keep them on hand. I ordered some of the red, green and blue output transistors, but what I got doesn't match what's installed. What I have are what looks like round metal canisters with aluminum heat sinks and a plastic piece over the top to hold them and warn about the voltage. What I got as replacements are TO-202 types. These replacements might be functional replacements, but not physically. They obviously generate enough heat to need a sink, so these can't be installed or they'll overheat.

I'm attaching a pic and if you look to the right of the RGB outputs, you'll see the type of transistor they are.

Aw geez, here we go again - part numbers, please? Original vs replacements.

And "replacements" - do you mean ECG/NTE or Zenith? You have to be a bit more specific to help some of us help you.

Cheers,

They are 121-868 and NOS Zenith parts. I was more telling DaveWM about it than asking anyone to fix something for me.

What are the original part numbers? 121-868's are TO-202, not what you have installed in the set (TO-5's, probably 121-775 or 121-777s, IIRC). Zenith made a mod kit that modded the video outputs to the 868's. You seem to have received the replacements for the 868s, but without going through the mod. Were 868's what you ordered? With the original number, I can look up the mod kit for you. The mod kit has the 121-868 transistor, the proper heatsink and that gooey heat sink compound. The sams I have lists the 868s, but not the original part numbers, you have to get that directly off the transistors.

If your set is operating ok, there is no need to do the mod.

Cheers,

Oh yeah, she working fine. I've just been trying to stock up on some parts so in the future I can handle anything that comes my way. I've been trying to get my hands of this TV for years and now that I finally have one, she's staying with me for a long time. :)

I'll be opening her up on Monday or Wednesday and I'll pull the sink off of one and get the number.

Now I could use a little assistance. Do you know how to increase the brightness range? I've done the adjustment of the brightness limiter and it's dead on. Only thing is, I have to crank the brightness almost all the way up to get it to where it's enough. I've never had a set where there was so little headroom to go higher and I don't think the limiter I set is the issue. I'm thinking I may need to crank up the G2s a little more?

Anyway, thanks for the input. I figure if the transistors for the outputs are working, and the replacements aren't actually going to make an improvement worth the effort, I'll just be patient and find OEMs of the current ones.

Looks like what's installed are the 121-743 transistors. Do you know which are newer/better?

Are the installed ones bad? If so, then you'll need the kit - I'll have to look them up in my looseleaf Zenith parts guide from 1980.

If they are good, and you want spares for the future, you'll also need the kit.

How did you get 121-868 from 121-743? Sams? A bit of advice - Sams is pretty broad with part numbers vs actual parts. I've got a Sams for a Packard Bell 19" Solid State set that has the same part number for every transistor in the sweep circuits - Wrong! The lesson is this - if it works, let it be. If you need parts, always quote the original part number, and never from Sams. If you had the Zenith CM (service manual), maybe, but never quote part numbers from Sams. Also, post the part numbers here in the forum (classifieds are best) to see what can be had for postage. As Sandy says, we may have a screw loose for wanting to preserve these old TVs, but we have each other's back nonetheless.

I'll report back tomorrow with the replacement info from Zenith.

Cheers,

Just wanted spares. And yes, I went by Sams. So far it hasn't been wrong except for the three video output transistors. So I have three of the 868s, but no sinks, etc. Not a huge deal as the current ones seem to be fine.